1. Field of the Invention
The present invention generally relates to ultrasonic imaging method and system with a dual display mode. More specifically, the present invention is directed to ultrasonic imaging method and system capable of displaying both of a B-mode ultrasonic image of a biological body under medical examination and a color Doppler image (or color flow mapping image) thereof over a wide field even in a deep interior portion by freely setting a beam steering direction.
2. Description of the Prior Art
Various types of ultrasonic imaging systems with a dual display mode, capable of displaying both of a B-mode image and a CFM image on a single display screen in a superimpose mode have been developed. The conventional ultrasonic imaging systems employ, for instance, a linear ultrasonic probe to perform either the linear scanning mode, or the sector scanning mode. In the linear scanning mode, a tissue section within a biological body under medical examination is scanned by ultrasonic beams positioned parallel to each other to produce a B-mode image and also a color blood-flow image (or CFM image) of the scanned interior tissue. In the sector scanning mode, the ultrasonic beam scanning is carried out for the interior tissue from a predetermined point in a radial form to produce these images.
Typically, the Doppler method has been widely utilized to acquire such a color blood-flow image, or a color flow mapping (CFM) image. The basic blood-flow measurement by Doppler method will now be summarized with reference to FIG. 1. In FIG. 1, an ultrasonic probe 1 is employed to project ultrasonic beams 2 therefrom to an interior portion of a biological body 3 under medical examination having blood vessels 4. The frequency of the ultrasonic beams 2 is selected to be "f.sub.o ". When the ultrasonic beams 2 having the frequency of "f.sub.o " collide with red blood cells 5 which are moving at a velocity of "v", echoes having a frequency of "f.sub.o '" are reflected from these red blood cells 5 due to the Doppler effect. That is, assuming now that the transmission frequency of the ultrasonic beam 2 is "f.sub.o ", the reception frequency of the echo is "f.sub.o '", and an incident angle between the ultrasonic beam 2 and the blood flow direction is ".theta.", the velocity of the blood flow "v" is calculated based on an equation (1 ): ##EQU1## where symbol "fd" indicates a shifted frequency, and symbol "c" denotes the sound velocity.
Accordingly, a frequency shift is proportional to cosine of the angle intersecting between the ultrasonic beam 2 and the blood flow direction as shown in the equation (1). When the ultrasonic beam 2 is positioned in parallel to the blood flow, the frequency shift becomes maximum, so that the frequency shift can be measured at high precision. When the ultrasonic beam 2 intersects with the blood flow at a right angle (90.degree.), the frequency shift becomes zero, so that this Doppler frequency-shift measurement can not be performed. Generally speaking, since blood vessels 4 are located parallel with a surface of the biological body 3, when the ultrasonic beams used for the color flow mapping are transmitted toward the surface of the biological body 3 at the right angle, these ultrasonic beams intersect with the blood flow direction at 90.degree.. As a result, the blood flow velocity cannot be measured (namely, cosine 90.degree.=0).
As the possible conventional methods for avoiding that the CFM measuring beam 2 intersects with the blood-flow direction at a right angle, there are two typical ultrasonic beam scanning methods. A first conventional beam scanning method is shown in FIG. 2. In FIG. 2, B-mode imaging ultrasonic beams 6B are transmitted from a probe surface 1S of the probe 1 at a right angle, whereas CFM imaging ultrasonic beams 6C are transmitted from this probe surface 1S at a certain inclined angle. In this linear scanning method, a rectangular-shaped B-mode image is displayed with a CFM image of a portion of this rectangular shape in a superimpose mode. FIG. 3 represents a second conventional beam scanning method in which both of B-mode imaging ultrasonic beams 8B and CFM imaging ultrasonic beams 8C are transmitted at a certain inclined angle. As a result, a color blood flow (CFM) image having a shape of parallelogram is displayed.
These B-mode/CFM-mode beam scanning methods are known from, for instance, U.S. Pat. No. 5,014,710 issued on May 14, 1991 to Maslak et al., entitled "STEERED LINEAR COLOR DOPPLER IMAGING", and Japanese Laid-open (KOKAI DISCLOSURE) Patent No. 62-227335 opened on Oct. 6, 1987.
On the other hand, in order to easily specify a position of a CFM image on a monitor screen, this CFM (color blood-flow) image is superimposed on a B-mode image in the conventional ultrasonic imaging systems. Accordingly, in the above-explained first linear scanning method shown in FIG. 2, only a color blood-flow image of a portion 7 where the B-mode imaging beams 6B transmitted from the probe surface 1S at a right angle intersects with the CFM-mode imaging beam 6C transmitted at a certain inclined angle can be displayed on a monitor screen. As easily understood from such a limited CFM image portion 7, since the area 7 of the CFM image is especially narrowed at the deep portion, it is difficult to diagnose either a kidney, or a liver located in a deep portion of a biological body, while observing such a narrow CFM image.
To improve difficulties of the narrow CFM image portion 7, the second conventional linear scanning method as shown in FIG. 3 has been proposed in which both of the B-mode imaging beam 8B and the CFM-mode imaging beam 8C are transmitted from the probe 1 at the same inclined angle with respect to the probe surface 1S, i.e., a surface of a biological body. Accordingly, a color blood-flow image having a shape of a parallelogram is displayed whose area is wider than the above-described CFM image area 7 of the first ultrasonic imaging method.
However, this second ultrasonic imaging method has the problems. Since only the color blood-flow image located oblique from the probe 1 is displayed, it is rather difficult to grasp where a curing portion (CFM-scanned portion) is actually located within the biological body. In particular, when a deep portion of the biological body is diagnosed, a great positional difference may be induced, which will deteriorate correct positioning precision for such a curing portion in the ultrasonic diagnose.